1. Field of the Invention
The present invention generally relates to variable speed transmissions and, more particularly, relates to disengagement mechanisms for variable speed transmissions.
2. Description of the Related Art
Variable speed transmissions, such as hydrostatic transmissions transmit rotary mechanical motion, typically from an internal combustion engine, to fluid motion, then back to rotary mechanical motion to rotate a drive axle in order to drive a vehicle, such as a lawn and garden tractor. By varying the fluid pressure, the hydrostatic transmission can regulate the output rotary mechanical motion such that varying output speeds in the forward and reverse directions are possible with a single speed engine. A radial piston type hydrostatic transaxle is disclosed in U.S. Pat. No. 5,373,697, entitled HYDRAULIC FLUID SYSTEM AND DUMP VALVE MECHANISM FOR A HYDROSTATIC TRANSAXLE, which is specifically incorporated by reference herein.
One common application of a hydrostatic transmission is within a riding lawn mower. Although lawnmowers are self-propelled, in many situations such as positioning the mower within a garage, or a retail sales space, it is necessary to manually push the mower. Hydraulic transmissions, however, present a special problem when the vehicle is to be manually moved. With gear transmissions, a neutral position is achieved by mechanically disengaging the input shaft from the output shaft by either disengaging the gears coupling the input shaft to the output shaft, or disengaging the output shaft from the gears. Thus it is possible to manually move the vehicle without resistance from the transmission system either from the output shaft or input shafts as they are disengaged from one another.
Unlike gear transmissions however, in a hydraulic transmission, the hydraulic motor forming part of the hydrostatic unit and its output remain hydraulically engaged with the fluid pump of the transmission even when in a neutral position and thereby presents substantial resistance to movement. Thus, when trying to push a vehicle having a hydrostatic transmission, even when in neutral, the output acts as the input, and consequently the motor will attempt to pump hydraulic fluid back towards the pump. This problem is accentuated under cold temperature conditions because the hydraulic fluid will become especially viscous.
U.S. Pat. No. 5,373,697 discloses a hydrostatic transmission with a disengagement mechanism which opens check valves within the hydrostatic transmission to thereby shortcircuit passages which connect the hydraulic pump and motor. The hydraulic fluid will therefore not be pumped back into the pump. This type of disengaging mechanism is often termed a "relief" valve or "dump" valve since it relieves or dumps the hydraulic fluid from the fluid conduit connecting the pump and the motor into the chamber surrounding the hydrostatic transmission within the hydrostatic transaxle housing.
Although such "dump valves" open valves so that hydraulic fluid can move from the motor side to the pump side, the fluid is somewhat viscous and moves from one side to the other slowly and with difficulty. This is especially a problem under cold conditions when the viscosity of the hydraulic fluid increases significantly.